The present invention relates to ovenable cartons prepared from paperboard, and more particularly to an easy opening lid for such a carton. The lid disclosed herein is particularly useful for tray type cartons, or cartons which include container trays having an upper integral flange. The lids of such cartons are glued or adhered to the flanges of the trays after filling, to close the cartons for shipment and storage. This step is usually accomplished with the use of a heat sealable coating applied to the inner surface of the lid and the inner surface of the tray including the flange. The coating also serves to make the paperboard waterproof and provides a food contact surface. However, when such cartons are heat sealed, the bond between the lid and flange areas of the tray is difficult to break. Thus, a variety of opening schemes have been proposed for such cartons including means for venting the cartons for cooking.
U.S. Pat. Nos. 4,312,451 and 4,531,668, owned by the present assignee herein, disclose two different schemes for opening cartons of the tray and lid type wherein the lids are heat sealed to tray flanges. In the '451 patent, a weakened score line is applied around the periphery of the lid where it is joined to the tray, or along the score line connecting the flange to the tray side walls, to produce a delamination area between the flange and the lid in the flange area when the opening tab is lifted. In the '668 patent, the lid includes a pair of continuous parallel weakened score lines, one on the inner surface of the lid, and another on the outer surface of the lid, each located inwardly from the periphery of the lid at distances spaced from the flange area of the tray. This produces a delamination area totally within the lid itself so as to leave portions of the lid still adhered to the tray flange for extra strength when the carton is opened. Meanwhile, U.S. Pat. No. 4,871,071 shows a venting and opening means for a lid integral with a flanged tray wherein the lid includes a combination of weakened score lines on the inner and outer surfaces of the lid at one corner. In the '071 patent and the '668 patent, the opening scheme requires that the lid be pushed inwardly at the corner to initiate the opening sequence. This action is not nearly as desireable as a lifting step to initiate the carton opening sequence.
Therefore, according to the present invention, a novel arrangement of weakened score lines are applied only to the inner surface of a paperboard lid to produce an easy opening means that permits the lid to be lifted away from the tray for venting and ultimately opening the carton.